Field of the Invention
The invention relates to a silicon semiconductor body, which can be soldered to a metal substrate plate via a succession of metal layers, and which before the soldering, in order from the silicon to the substrate plate, has an aluminum layer and a diffusion barrier layer.
Such semiconductor bodies are incorporated into semiconductor elements, especially power semiconductor elements, which are on the market in great numbers. The sequence of metal layers as a rule includes an aluminum layer, which is seated on a silicon semiconductor body. The aluminum layer adheres well to the silicon and form a perfect ohmic contact, especially with p-doped silicon. In the prior art, a diffusion barrier layer, which is usually of titanium or chromium, is seated on the aluminum layer and acts as an adhesion promoter and back-side barrier between a further metal layer, as a rule a nickel layer, seated on the diffusion barrier layer, and the aluminum layer.
Because the coefficients of thermal expansion are different for the individual metal layers on the one hand and for the silicon semiconductor body on the other, severe mechanical stresses are engendered. Especially in thin semiconductor bodies, that is, semiconductor bodies that have a thickness of 250 .mu.m or less, severe warping of the wafer occurs, that is, warping greater than 1000 .mu.m.
This makes handling of the wafers more difficult; increased cassette positioning mistakes occur, and there is an increased danger of breakage. Until now, the attempt was made to solve the problem by minimizing the nickel layer thickness as much as possible, so that the soldering still had adequate adhesion strength. Despite reduced nickel layer thicknesses, that is, layer thicknesses of approximately 1 .mu.m, nevertheless in production wafer warping of 700 to 2000 .mu.m still occurs and leads to the above problems.
Particularly with a view to the desire for ever-thinner semiconductor bodies, that is, semiconductor bodies that have a thickness of approximately 100 .mu.m, there is a need for a metallizing process that helps solve the above problems. Such semiconductor bodies are needed particularly in power field effect transistors and IGBTs of the vertical type.
Published, Non-prosecuted German Patent Application DE 38 23 347 A1 describes a semiconductor element having a high current carrying capacity and a contact layer construction of the semiconductor body. The metallizing includes a first layer of aluminum, a second layer of chromium or titanium as an adhesion layer and as a diffusion barrier for the aluminum, a solderable third layer of nickel, and a concluding protective layer of gold or palladium, or of a solderable layer each with a partial layer of nickel and copper. The copper at the same time is the outermost layer or can also be covered with gold or palladium.
In IEEE Transactions on Electron Devices, 1986, Vol. ED-33, No. 3, pages 402-408, a silicon power transistor with a stepped electrode structure and a titanium nitride diffusion barrier is described. The titanium nitride diffusion barrier layer is applied, in the form of a succession of titanium, titanium nitride, and titanium, between a gold electrode terminal and a silicon substrate. As a result, high reliability and a long service life of the bond are attained, and a gold-silicon reaction is prevented.